There are numerous recognized methods for dyeing and/or incorporating additives into fibers and other fibrous materials, consisting primarily of methods which utilize organic solvents such as dimethylsulfoxide ("DMSO"), dimethylacetamide ("DMAc"), dimethylformamide ("DMF"), N-methylpyrrolidone ("NMP"), acetophenone, acetanilide and methyl benzoate. Exemplary systems utilizing these organic solvents have been proposed in U.S. Pat. Nos. 3,558,267 to Langenfeld, 3,741,719 to Ramanathan et al and 4,525,168 to Kelly, and U.K. Patent Nos. 1,275,459 to Gruen and 1,282,113 to Burtonshaw et al.
One particular solvent which has been proposed is N-cyclohexyl-2-pyrrolidone ("CHP"). Processes utilizing this particular solvent are proposed in U.S. Pat. No. 4,073,615 to Lacroix et al which utilize a glycol or glycol ether in combination with N-cyclohexyl-2-pyrrolidone to dye polyamide fibers, and more recently in U.S. Pat. No. 4,898,596 to Riggins et al to dye poly(m-phenyleneisophthalamide) and polybenzimidazoles.
Despite the general availability of dyeing processes using solvents including N-cyclohexyl-2-pyrrolidone, there continues to be a desire for process improvements which enhance the dyeability of fibers, for example, by permitting the substantially full exhaustion of dyes and additives such as flame retardants into the fibers. This is particularly important in difficult to dye fibers, for example, aromatic polyamides such as poly(m-phenyleneisophthalamide) ("Nomex.RTM.") and poly(p-phenyleneterephthalamide) ("Kevlar.RTM."), polybenzimidazoles ("PBI") and aromatic polyimides.
In addition to the general desire for process improvements to enhance dyeability, the prior art also acknowledges that there is a continuing need for any system which would be effective to and uniformly dye Kevlar.RTM. fibers and blends of Kevlar.RTM. with other fibers. This problem is described, for example, in "Evaluation of the STX.RTM. System for Solvent Dyeing of Industrial Fabric Part II: Kevlar.RTM. Aramid and PBI.RTM. Fabrics", Cook et al, Journal of Industrial Fabrics, vol. 2, No. 1, Summer 1983 and U.S. Pat. No. 4,898,596 to Riggins et al. This problem is particularly acute for the most commonly available Nomex.RTM. product, namely Nomex.RTM. T-455. Nomex.RTM. T-455 includes 5 percent Kevlar.RTM., and despite the availability of methods for dyeing 100 percent Nomex.RTM. there are no effective methods for uniformly dyeing the Kevlar.RTM. fraction of Nomex.RTM. T-455.
It is therefore an object of the present invention to provide an improved method for dyeing fibrous materials, and particularly to provide a method which provides particularly advantageous results, for example, more uniform dyeing, when used with difficult to dye fibers such as aromatic polyamide fibers including Kevlar.RTM. fibers, polybenzimidazole fibers, aromatic polyimide fibers, copolymers of the monomers thereof and blends thereof.